There have been various uses of spectroscopy for characterizing or understanding the make-up of materials. A test technique for distinguishing wool from polyester-fibre and cotton fabrics employing photoacoustic spectroscopy is discussed in the article, Davidson and King, "A Method of Distinguishing Wool from Polyester-fibre and Cotton Fabrics", J. Text. Inst., vol 74, p. 382 (1983). This technique makes use of the absorption spectrum of the fabric in the near infrared, which is the complement of the reflection spectrum. A major disadvantage of the proposed technique is the inability to control the resonant cell's sample wall in real-use conditions.
Another article, Derkson and Monahan, "A Reflectometer for Measuring Diffuse Reflectance in the Visible and Infrared Regions", J. Opt. Soc. Am., Vol. 42, p. 263 (1952), discusses an instrument for measuring diffuse reflectance of materials, and employs as targets, among other things, cotton twill and wool serge. This instrument operates in the infrared, and in the visible-wavelength range for visual appearance purposes.
Another instrument is reported by Timo Hyvarinen, "Rugged Multiwavelength NIR and IR Analyzers For Industrial Process Measurements", SPIE Conference Proceedings, Vol. 1266, p. 99 (1990), European Congress on Optics, The Hague. This device is a hand-held, battery-operated meter for optically measuring the moisture content of peat. A tube is inserted into a peat bog, and a trigger is pulled in order to display the moisture measurement on a display. This device uses a miniature tungsten lamp, and a detector including two side-by-side lead-sulfide (PbS) photoconductive cells located behind interference filters peaked at 1.80 and 1.94 microns, the useful sensing bands for water. The PbS cells and interference filters are mounted in a windowed, N.sub.2 -filled, hermetically-sealed container, to prolong filter life. The light source is electrically modulated at 35 Hz and transmitted through a quartz window into the peat, and the radiation diffusely reflected by the peat is measured by the detector, which provides output signals indicative of the moisture content of the peat.
It would be desirable to provide a method and apparatus for identifying unknown diffusely-reflecting materials, such as textile fabrics, which preferably use optical means to sense and rapidly identify the unknown material. Although the prior art discloses devices for optically measuring a characteristic or the make-up of a diffusely-reflecting material, it has failed to provide a method and apparatus for identifying an unknown material rapidly and in real time, which preferably uses infrared spectroscopy for identifying an unknown material based on the characteristic spectrum of that material.